Sigma delta modulators have achieved popularity for use in integrated circuit devices for both analog to digital and digital to analog conversion. Their attractiveness for integrated circuit systems is due in part to the fact that they employ a one bit digital to analog converter that is inherently perfectly linear and does not require precision component matching. However, single bit digital to analog converter performance is not sufficient to accomplish faster and higher resolution conversion required for state of the art devices. Though a one bit digital to analog converter achieves the desired linearity, accuracy and stability suffer as compared to multi-bit digital to analog converters. The quantization noise, which depends on the number of bits of precision in the analog to digital and digital to analog converters of the sigma delta modulator, decreases as more bits of precision are used in the converters.
The linearity of the resulting oversampled multi-bit system is essentially limited by the linearity of the digital to analog converter alone. The performance of multi-bit converters is generally degraded by internal digital to analog converter nonlinearity due to component mismatch. For high accuracy multi-bit digital to analog converters, off chip elements or trimming are needed to perform precision component matching in order to obtain the desired linearity. In conventional integrated circuit technology, a multi-bit digital to analog converter can only be built with inherent linearity of about ten bits. It is thus desirable to have an oversampling sigma delta modulator system with both quantization noise and linearity areas below the sixteen bit level and reduce degradation due to component mismatch. Such systems have achieved such a level with higher order modulators, but these higher order modulators have suffered from instability problems.
From the foregoing, it may be appreciated that a need has arisen for a digital to analog converter which achieves the advantages of accuracy, stability, and linearity. A need has also arisen for a digital to analog converter that performs precision component matching particularly suited to oversampled data systems. Further, a need has arisen for a second order sigma delta modulator that provides high resolution and reduces the effect on linearity caused by component mismatching.